1. Field of the Invention
The invention relates to an implantable apparatus for the early diagnosis and suppression of tachycardia in the heart.
2. Background Art
As for the background of the invention, a short explanation is necessary of the cardiac arrhythmias designated as tachycardia, in which the heart rate rises beyond a physiologically reasonable degree. In this case, the individual contractions of the heart take place so fast that no sufficient quantity of blood is transported per contraction. The heart rate increase is over-compensated by the decrease in volume of the heartbeat.
Fundamentally, ventricular tachycardia conditions are dangerous, since the pumping capacity of the heart is mainly performed by the ventriculus, while the atrium contributes to blood conveyance only to a very limited degree.
Conditions of ventricular tachycardia can be subdivided in varying degrees of severity. A first stage is characterized by changes in the morphology of the electric stimulus signals of the cardiac muscle cells and by irregularity in the slightly increased cardiac rhythm.
A second stage is the so-called ventricular flutter at a heart rate that has not yet risen to a critical degree. The decrease of the pumping capacity can however lead to a patient's syncope.
The most serious stage is the so-called ventricular fibrillation, in which case the drastic decrease of the pumping capacity leads to the death of a patient within a very short time.
To date, various measures have been taken for tachycardia therapy, adapted to the degree of severity of tachycardia. At a less critical stage, the setting in of tachycardia is frequently suppressed by so-called antitachycardia stimulation at amplitudes of some volts which is customary for heart pacemakers. The heart beating stably but too rapidly is "caught up", to which end stimulation pulses at certain time intervals are used. The prerequisite for the success of such a moderate therapy is the timely recognition and classification of such a condition of tachycardia.
In the case of ventricular fibrillation, the only thing that works is as a rule a defibrillation shock of some hundreds of volts, if this potential can be applied directly in the heart. Defibrillation shocks applied extracorporally--for example via chest electrodes--even need amplitudes of some kV.
If successful, a defibrillation shock leads to electric excitation of all cardiac muscle cells so that subsequently, all the cells find themselves in the so-called refractory phase. During this refractory phase, which is to be designated as a "dead period", any stimulus conduction within the heart is interrupted, even that one previously leading to fibrillation. The latter may base on circulating self-excitation, in which an excitation front circulates cyclically around a pathologically disfigured spot in the cardiac tissue--for instance an infarction scar.
Summing up, it may be said that the defibrillation shock is sort of a "reset" of the entire cardiac muscle apparatus which is very drastic and painful to the patient. Therefore, any effort should be made for conditions of tachycardia to be diagnosed as early as possible so that measures can be taken to prevent fibrillations from setting in.
However, the detection of high heart rates alone is not sufficient for this early diagnosis, since, in particular in the range of low heart rates, tachycardia conditions cannot be distinguished from natural increases of the heart rate as a result of physical stress. This is why the prior art mainly proceeds from two objectives for the early diagnosis of tachycardia. On the one hand, this consists in the evaluation, implemented on a heart pacemaker, of the so-called heart rate variability in ECG diagrams. Irregularities or certain acceleration patterns of the cardiac rhythm are to be recognized, which occur regularly at the beginning of the virtual tachycardia condition. Drawbacks of this method reside in the comparatively high requirements of computing and storing capacity, which today's implants cannot comply with sufficiently.
On the other hand, early tachycardia diagnosis makes use of an evaluation of the action potentials of cardiac muscle cells. Their morphology is subject to certain modifications already very early before the beginning of tachycardia. However, the implementation of such evaluation methods is not put into practice for lack of adequate findings, and presumably, the requirements on the computing and storing capacities of today's standards will again be too high for it to be possible to implement the corresponding method on an implant.